1. Field of the Invention
The present invention relates to a laser frequency stabilizing apparatus, a method and a computer program product for stabilizing a laser frequency by varying a resonator length on the basis of an optical output signal.
2. Description of the Prior Art
A wavelength of a solid laser of a 532 nm range based on continuous wave oscillation can be used as a length standard. In the continuous wave oscillation, an Nd:YAG crystal or the like that can be pumped by a semiconductor laser is used as a gain medium. As a matter of fact, in order to measure a length using a wavelength of laser light, the laser is required to have a single frequency. In other words; the laser light is required to oscillate with a single longitudinal mode. Moreover, in order to stabilize a frequency of the laser light with the use of a spectroscopic technique for atomic or molecular absorption lines, an oscillation frequency must be selected arbitrarily.
When length measurement is carried out using the wavelength of the laser light, there is a need to heighten frequency stability of the laser light in order to reduce uncertainty of length measurement. In an iodine stabilized laser using a spectroscopic technique by absorption lines of iodine molecules, high frequency stability of the laser light can be achieved by controlling an oscillation frequency in the vicinity of the center of a saturated absorption signal (See Patent Document 1: Japanese Patent Application Publication No. 2001-274495, Patent Document 2: Japanese Patent Application Publication No. 10-163549, and Patent Document 3: Japanese Patent Application Publication No. 2000-261092).
A plurality of iodine saturated absorption lines normally exist, and in the iodine stabilized laser, a desired saturated absorption line is to be searched from the plurality of iodine saturated absorption lines to stabilize (or lock) an oscillation frequency.
In consideration of such necessity, the Patent Document 1 discloses a system for identifying a saturated absorption line on the basis of an interval between the saturated absorption lines as a method of automatically stabilizing an oscillation frequency at a desired saturated absorption line.
However, since the system of Patent Document 1 measures an interval between the saturated absorption lines, the system is required to include measuring means capable of measuring the interval between the saturated absorption lines. An instruction value (applied voltage to a piezoelectric element) of an actuator (including the piezoelectric element) may simply be utilized to the measuring means. However, in such a case, the actuator must have sufficient accuracy of linearity relative to interval measuring of the saturated absorption lines. Namely, if the system of Patent Document 1 is utilized, the system is required to have high accuracy for measurement. For this reason, the system is to be expensive and cannot be realized without difficulty. Further, in the case of an apparatus using an actuator having large elapsed-time variation in the system of Patent Document 1, locking (or stabilizing) of an oscillation frequency cannot sufficiently follow the elapsed-time variation.